(im usually on the diet boards, so please dont mind me barging in....just thought i'd check out some others, and i found your post interesting.)
Mostly, b/c....Im divorced, have 2 kids, live on social security from my ex, who passed away, and my fiance is michael

(heehee).
Here's what we do. If its my family wedding, I pay. If its his, he pays. (same for friends...unless its mutual friends, in which case, he usually does...just b/c he has more money than i do
If your girlfriend already stated she wants to pay, let her. You may want to ask one more time "are you sure, sure, you have this?"...after that, let her have her pride! single mothers are very proud of themselves when they are able to keep up
Enjoy the wedding!